The invention relates generally to induction machines and more particularly to sensorless rotor tracking of induction machines.
The elimination of rotor shaft transducers such as rotor position and speed sensors is highly desirable to reduce cost, to reduce total motor package size, and to improve system reliability. Rotor shaft transducers tend to be a major source of failure and expense in ac machine drives.
Jansen et al., U.S. Pat. Nos. 5,585,709 and 5,565,752, for example, describe a method of control whereby high frequency signal injection is used to track the rotor angular position and velocity of an induction machine having a rotor providing an impedance that varies with rotor position. Various means are presented whereby an induction machine rotor can be modified to introduce position dependent impedances that are detectable from the stator terminal via signal injection.
Degner et al., xe2x80x9cUsing Multiple Saliencies for the Estimation of Flux, Position, and Velocity in AC Machines,xe2x80x9d IEEE Industry Applications Society Annual Meeting, New Orleans, La., 1997, pp. 760-767, describes the tracking of rotor position and velocity of an induction machine with an unskewed rotor via detection of impedance variations due to rotor slotting. In most induction machines smaller than 1000 hp, the rotor slots are skewed causing the impedance variation due to slotting to be nearly undetectable with the resolution of standard current and/or voltage feedback sensors currently used in motor drives.
Zinger et al., xe2x80x9cA Direct Field-Oriented Controller for Induction Motor Drives Using Tapped Stator Windings,xe2x80x9d IEEE Transactions on Power Electronics, 1990, pp. 446-453, states that the effect of slotting is greatly enhanced in the neutral voltage of WYE (Y or star) connected machines. According to Zinger et al., by summing the three instantaneous line-neutral (phase) voltages, the fundamental voltage components are removed via cancellation, leaving the zero sequence components which include saturation-induced, slot-ripple-induced, and inverter-induced zero sequence voltages. A phase-locked loop was described as being used to lock onto the slot-ripple-induced zero sequence voltage and provide a rotor speed estimate. Because both the amplitude and frequency of the slot-ripple-induced voltage were proportional to speed, the method was not successful at low speeds. Furthermore direction of rotation could not be determined because quadrature signals were not available and the frequency of the tracked slot-ripple-induced voltage was centered about zero.
A number of alternative methods of tracking rotor velocity have been investigated using fast Fourier transform (FFT) based approaches with one example being Hurst et al., xe2x80x9cSpeed Sensorless Field-Oriented Control of Induction Machines Using Current Harmonic Spectral Estimation,xe2x80x9d IEEE-Industry Applications Society Annual Meeting, 1994, pp. 601-607. The fundamental limitation of these approaches is the poor dynamic performance resulting from the requirement of numerous cycles to be sampled by the FFT method, thus precluding operation near zero speed and achieving high dynamic performance control.
Holtz, xe2x80x9cSensorless Position Control of Induction Motorsxe2x80x94An Emerging Technology,xe2x80x9d IEEE Transactions on Industrial Electronics, Vol. 45, No. 6, December 1998, pp. 840-852, describes dynamic tracking of rotor position and velocity via tracking of the rotor slot impedance variation through the combination of special inverter PWM voltage switching and voltage measurement sampling utilizing the zero sequence component of a WYE connected machine. The main limitations of this method are the requirement for a high number of voltage measurement samples per PWM inverter switching cycle, a special inverter PWM switching pattern that introduces additional torque ripple and losses into the machine, and the need for an additional neutral voltage connection and voltage sensor compared with conventional motors and drives.
Hammerli et al., xe2x80x9cA Rotor Speed Detector for Induction Machines Utilizing Rotor Slots Harmonics and an Active Three Phase Injection,xe2x80x9d Proceedings of EPE""87, 1987, pp. 599-604, describes a method of rotor speed detection utilizing rotor slot harmonics and active three-phase signal injection in WYE-connected machines and states that speed can be detected over the range of 2% to 100% of the nominal rotor speed but not at near zero speeds.
It would therefore be desirable to have a system and method enabling accurate rotor velocity and position tracking of polyphase induction machines without need for rotor position shaft sensors (that is xe2x80x9ctachlessxe2x80x9d or xe2x80x9csensorlessxe2x80x9d control) including during near zero frequency and velocity operating conditions.
Briefly, in accordance with one embodiment of the present invention, a drive system comprises a rotor position and velocity tracker adapted to decouple fundamental frequency effects of a zero sequence signal of an induction machine from the zero sequence signal and to use a resulting error signal to estimate a position and a velocity of a rotor of the induction machine.